An inkjet printhead (to be referred to as a printhead hereinafter), which generates thermal energy by sending an electric current to a heater arranged in the nozzle so as to discharges ink, has conventionally been known.
This printhead is a printhead which employs a method of bubbling ink near the heater by using the generated thermal energy, and discharging ink from the nozzle to print.
In order to print at a high speed, heaters (printing elements) mounted in a printhead are desirably concurrently driven as many as possible to discharge ink at the same timings. However, due to the limited capacity of the power supply of a printing apparatus having the printhead and a voltage drop caused by the resistance of a wiring line extending from the power supply to the heater, a current value which can be supplied at once is limited. For this reason, a time divisional driving method of time-divisionally driving a plurality of heaters to discharge ink is generally adopted. For example, a plurality of heaters are divided into a plurality of groups, and time divisional control is so executed as not to concurrently drive two or more heaters in each group. This can suppress a total electric current flow through heaters and eliminate the need to supply large power at once.
FIG. 17 is a circuit diagram showing an example of the arrangement of a heater driving circuit mounted in a conventional inkjet printhead.
The heater driving circuit shown in FIG. 17 is configured by mounting x heaters in each of m groups so as to concurrently drive one heater in each group, i.e., a total of m heaters, perform this operation x times, and complete driving of one cycle.
As shown in FIG. 17, MOS transistors 1102-11 to 1102-mx corresponding to respective heaters 1101-11 to 1101-mx are divided into m groups 1100-1 to 1100-m which contain the same number of (x) MOS transistors. More specifically, in the group 1100-1, a power supply line from a power supply pad 1103 (power source terminal) is commonly connected to the heaters 1101-11 to 1101-1x, and the MOS transistors 1102-11 to 1102-1x are series-connected to the corresponding heaters 1101-11 to 1101-1x between the power supply pad 1103 and ground (GND) 1104.
When a control signal is supplied from a control circuit 1105 to the gates of the MOS transistors 1102-11 to 1102-1x, the MOS transistors 1102-11 to 1102-1x are turned on so that an electric current can flow from the power supply line through corresponding heaters and the heaters 1101-11 to 1101-1x are heated.
FIG. 18 is a timing chart showing a timing at which an electric current is sent to drive heaters in each group of the heater driving circuit shown in FIG. 17. FIG. 18 exemplifies the group 1100-1 in FIG. 17.
In FIG. 18, control signals VG1 to VGx are timing signals for driving the first to x-th heaters 1101-11 to 1101-1x belonging to the group 1100-1. More specifically, the control signals VG1 to VGx represent the waveforms of signals input to the control terminals of the MOS transistors 1102-11 to 1102-1x of the group 1100-1. A corresponding MOS transistor 1102-1i (i=1, x) is turned on for a high-level control signal, and a corresponding MOS transistor is turned off for a low-level control signal. This also applies to the remaining groups 1100-2 to 1100-m. In FIG. 18, Ih1 to Ihx represent current values flowing through the heaters 1101-11 to 1101-1x. 
In this manner, heaters in each group are sequentially and time-divisionally driven by sending an electric current. The number of heaters driven in each group by sending an electric current can always be controlled to one or less, and no large electric current need be supplied to a heater.
FIG. 19 is a view showing the layout of power supply lines connected from the power supply pad 1103 to the groups 1100-1 to 1100-m shown in FIG. 17. In other words, FIG. 19 is a view showing part of the layout of a board (head substrate) which forms the heater driving circuit shown in FIG. 17. Particularly, FIG. 19 shows the layout of power supply wiring part in a case where heaters (not shown) are arranged on an upper side of this drawing paper.
As shown in FIG. 19, power supply lines 1301-1 to 1301-m are individually connected from the power supply pad 1103 to the respective groups 1100-1 to 1100-m, and power supply lines 1302-1 to 1302-m are connected to the ground (GND) pad 1104. In a printhead having m×x heaters (printing elements), time divisional driving of sequentially driving one printing element in each group requires m power supply lines and m ground lines.
As described above, by keeping the maximum number of heaters concurrently driven in each group to one or less, a current value flowing through a wiring line divided for each group can always be suppressed to be equal to or smaller than a current flowing through one heater. Even when a plurality of heaters are concurrently driven, voltage drop amounts on wiring lines on the heater substrate can be made constant. At the same time, even when a plurality of heaters belonging to different groups are concurrently driven, the amounts of energy applied to respective heaters can be made almost constant.
Recently, printing apparatuses require higher speeds and higher precision, and a mounted printhead integrates a larger number of nozzles at a higher density. In heater driving of the printhead, as many heaters as possible are required to be simultaneously driven at a high speed in terms of the printing speed.
A printhead substrate (to be referred to as a head substrate hereinafter) which integrates heaters and their driving circuit is prepared by forming many heaters and their driving circuit on the same semiconductor substrate. In the manufacturing process, the number of heater substrates formed from one semiconductor wafer must be increased to reduce the cost, and downsizing of the head substrate is also demanded.
When, however, the number of concurrently driven heaters is increased, as described above, the head substrate requires wiring lines corresponding to the number of concurrently driven heaters. As the number of wiring lines increases, the wiring region per wiring line decreases to increase the wiring resistance when the area of the head substrate is limited. Further, each wiring width decreases, and variations in resistance between wiring lines on the head substrate increase. This problem occurs also when the head substrate is downsized, and the wiring resistance and variations in resistance increase. Since heaters and power supply lines are series-connected to the power supply on the head substrate, as described above, increases in wiring resistance and variations in resistance lead to an increase in the variation of a voltage applied to each heater.
When energy applied to a heater is too small, ink discharge becomes unstable; when the energy is too large, the heater durability degrades. In other words, in a case where the variation of the voltage applied to heaters is large, the heater durability degrades or ink discharge becomes unstable. For this reason, to print with high quality, energy applied to a heater is desirably constant. Furthermore, it is also desirable to stably apply appropriate energy in view of the durability.
In the above-described time divisional driving where the number of concurrently driven heater is one or less, the voltage drop can be suppressed within the head substrate. However, since a wiring line outside the head substrate is common to a plurality of heaters of plural groups, the amount of voltage drop on the common wiring line changes depending on the number of concurrently driven heaters. In order to make energy applied to each heater constant against variations in the above voltage drop, energy applied to each heater is conventionally adjusted by the voltage application time. However, as the number of concurrently driven heaters increases, a current flowing through a common wiring line generates a large amount of voltage drop. As a result, the voltage applied to a heater decreases. The voltage application time in heater driving must be prolonged to compensate for the voltage drop, and this makes it difficult to drive a heater at a high speed.
As a method which solves such problems caused by variations in energy applied to a heater, for example, Japanese Patent Publication Laid-Open No. 2001-191531 proposes a method of driving a printing element by a constant current.
FIG. 20 is a circuit diagram showing a heater driving circuit disclosed in Japanese Patent Laid-Open No. 2001-191531.
In this arrangement, printing elements (R1 to Rn) are driven by a constant current using constant current sources (Tr14 to Tr(n+13)) and switching elements (Q1 to Qn) which are arranged for the respective printing elements (R1 to Rn).
However, constant current driving disclosed in Japanese Patent Publication Laid-Open No. 2001-191531 requires transistors equal in number to printing elements in addition to switching elements (Q1 to Qn). As a result, the area of the heater substrate becomes much larger than that in a conventional driving method, and the cost of the heater substrate becomes higher.
In order to stabilize energy applied to a heater, output currents from a plurality of constant current sources must be uniform. However, as the number of constant current sources increases, output currents from these constant current sources vary much more. It is difficult to reduce variations in output current between a plurality of constant current sources particularly on a head substrate having a greater number of heaters for higher speed and higher precision of printing in the printing apparatus.